This application is based on Japanese Patent Application No. 11-111494 (1999) filed Apr. 19, 1999, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to test pattern printing methods, and more specifically to a test pattern printing method to print a test pattern which is checked to detect a color deviation caused by variations in an output characteristic among several print heads, and an information processing apparatus and a printing apparatus to print the test pattern.
2. Description of the Related Art
As devices for automated office work have come into wide use, the necessity for producing color images has increased. Output devices for outputting color images are known, such as printing apparatuses of various methods, including ink jet, electro-photographic, and heat transfer methods. In recent years, these printing apparatuses have become able to output picture-like color images of relatively high quality.
Such printing apparatuses generally print color images using either three chromatic colorants, cyan (C), magenta (M) and yellow (Y), or four colorants with an additional achromatic color, black (K). The printed color image, however, may have a color deviation or color misrepresentation wherein the produced color in the printed image differs from a desired one due to an imbalance of output characteristics (e.g., reflection density or lightness, saturation, hue, etc.) among the print heads for the each respective color. In the ink jet system, for example, the print heads may have differences in an amount of heat produced by the heaters as a result of differing film thickness of the heaters, and/or differences in the size and shape of the ink ejection orifices, causing individuality in an amount of ejected ink. These differences, in turn, may result in the output characteristic imbalance among the print heads, as described above. Furthermore, these differences are not only inherent to print heads, but it is also known that these differences result over time.
To alleviate this problem of color deviation or misrepresentation, a personal computer functioning as a host computer for the printing apparatus, or the printing apparatus itself, may perform a correction process in which the host computer or printing apparatus detects the color deviation due to the differences among respective print heads and, based on the result of detection, corrects the output characteristics of the print heads to match predetermined characteristics. Such correction processing is generally carried out in two ways distinguished primarily on whether the detection of the color deviation is done by using an input device such as scanner or by a human visual check. An outline of these two correction methods, as distinguished by the way of detecting the color deviation, is briefly explained below.
The method using the input device such as scanner, for example, firstly prints a patch pattern by using the print heads of the C, M, Y and K coloring materials that are subjects of the correction, as disclosed in the official gazette of Japanese Patent No. 2661917. Then, the patch pattern is read by the scanner to detect a difference between a read value which represents an output characteristic of the head and an expected value calculated from the print data for the patch pattern. Based on the detected difference, the color deviation is corrected. The patch pattern used in this method may be a solid pattern of each of the C, M, Y, K colors or a patch pattern of each color having patches showing a gradation in density. The gradation patch pattern can also be used to detect an output characteristic with respect to a halftone, thus improving precision of detecting the color deviation. A method is also known that combines the C, M, Y and K to form second- or third-color patch patterns and thereby improves the detection and correction accuracy.
On the other hand, the method using the visual check, unlike the method using the input device, cannot easily detect an absolute value with respect to the output characteristic for each color head. Hence, a detection method that uses a third-color patch pattern printed by mixing three colorants C, M, Y is more commonly used. More specifically, a plurality of patches of almost gray color are printed. The plurality of patches include a central patch which is expected to be printed at a predetermined ratio of the three colorants (represented by print data of the three colorants) so as to be a patch of an achromatic color when printed by using print heads with an average ink ejection amount or without any deviation of ink ejection amount. The other patches are printed with their three-color ratios successively changed slightly. From the plurality of patches a user visually chooses one patch closest to the achromatic color so as to detect the patch of the most appropriate output characteristic balance among the C, M, Y color print heads. Then, the correction data corresponding to the detected patch is used to correct the output characteristic of each print head. This method of detecting the color deviation takes advantage of a fact that a slight imbalance in the output characteristics among the C, M, Y color print heads causes relatively large output characteristic changes in the patch, resulting in the patch deviating from the achromatic color.
While the problem of color deviation and the method of its detection have been described for the case of the print heads of ink jet system, differences in the output characteristics of the color print heads may also occur with printing apparatuses using electrophotographic or heat transfer systems due to causes peculiar to the principles of the respective printing systems. In these printing systems, also, the color deviation detection and correction are performed in the similar manner.
Unfortunately, the conventional color deviation detection method described above has several drawbacks.
First, in a typical environment in which the printing apparatus is used, the method using an input device such as a scanner requires that the user controls, has access to, or xe2x80x9cownsxe2x80x9d the input device. However, in a typical environment not all users have the input device available. Thus the method using the scanner or other input devices is not practical. Further, even if some input device is made available, the correction process on a basis of the color deviation detection using such wide variation of input devices is often very difficult to carry out.
The method based on visual check, on the other hand, does not require any special input device and thus can be employed by any user to detect color deviation. It is, however, not easy to select a patch closest to the achromatic color from the plurality of patches with their C, M, Y color ratios progressively yet slightly changed.
For example, JIS (JIS E3305, JIS Z8721, JIS L0600, etc.) and various other specifications concern a color difference (xcex94E). In these specifications, a range of the color difference of 3.2-6.5 is defined as a xe2x80x9crange that can be handled as the same color in terms of impression.xe2x80x9d This suggests that visually picking a patch closest to the achromatic color from patches in this range of color difference is difficult. It is also stated that the color difference in such a range may xe2x80x9ccause a customer to complain about difference in color when selecting paint color,xe2x80x9d suggesting that even in this range of color difference, if a wrong patch is selected, an image printed after being corrected based on a correction value corresponding to the selected patch may fail to provide a desired color.
For this reason, an effort is being made to improve an accuracy of the visual check-based color deviation detection method.
FIG. 1 is a view schematically showing an example of a test pattern that enables users to detect the color deviation with greater precision. In FIG. 1, each frame represents a patch printed with a mixture of C, M and Y colors and with K. The four numbers in each of two areas in each patch represent multi-valued gradation data for printing the associated area and correspond, from top to bottom, to C, M, Y and K. Thus, the patch shown in FIG. 1 consists of two upper and lower areas. A test pattern is made up of an array of such patches with their gradation value of M increasing vertically downward in five steps and that of C increasing horizontally toward right in five steps.
As is apparent from FIG. 1, a test area defined by the upper area in each patch is an area printed with a mixture of the C, M, Y colorants (the mixed color is hereinafter referred to as xe2x80x9cPCBkxe2x80x9d that is xe2x80x9cprocess blackxe2x80x9d). A reference area defined by the lower area is printed with only K, the colorant of achromatic color. In the upper area printed with PCBk the gradation value of Y is fixed at a 128 level. Further, as described above, value of C in the pattern increases toward the right and value of M increases downwardly.
With the test pattern constructed in this way, the precision of visual detection can be improved. More specifically, this pattern takes advantage of a visual characteristic that when objects are close together, a small color difference can be perceived. More concretely, this pattern is based on a fact that even if the color difference is as small as 0.8-1.6, comparison between adjacent objects allows a human eye to perceive the color difference. The pattern of this method allows the user, rather than to search, without any criterion, through a plurality of patches printed with only PCBk to find a patch closest to achromatic color, to instead make one-to-one comparison between an area printed with an achromatic color K and an area printed with PCBk which areas are adjacent to each other and thereby select a patch with a PCBk area having the least color difference from the area of achromatic color K. Thus, the precision of visual detection can be improved. This method is hereinafter called an xe2x80x9cadjacency comparison methodxe2x80x9d.
As described above, the precision of visual detection of a color difference (color deviation) can generally be improved by using the adjacency comparison method. However, when it is attempted to further improve the detection precision, particularly in the two-value or binary value printing, the conventional adjacency comparison method may prove insufficient. This problem will be explained in detail as follows.
The test area of PCBk in the conventional patch is generally printed not by what is called a solid printing in which dots of each colorant are formed on all pixels, but instead by half-tone printing using an intermediate gradation value of each colorant. The half-tone printing is used for a following reason. Considering that there are errors in the positions of the dots formed and that the dots are circular in actual printing, a printing system is designed to overlap the formed dots to ensure that during the solid printing the paper surface is filled with dots so that no areas are left unprinted. That is, when a printing duty is near 100%, the coverage rate of the paper surface with the dots is about 100%, which in turn does not allow the variation in a size of the formed dot and a position of the formed dot resulting from a change of the output characteristic to be detected as a significant change. Therefore, the PCBk area is not solid-printed for each colorant dot but is often printed with gradation data that corresponds to an intermediate value of the printing duty. More specifically, of 256 gradation values represented as data of 8 bits, the data close to a value of 128 is used for printing the PCBk area. As for an adjacent area printed with the K colorant, it is desired that a difference in lightness of the adjacent area from the above-described PCBk area is not so large. Thus, the data near the gradation value of 128 is also often employed in this case.
However, if each gradation value of PCBk in the test area is simply set near 128 as described above, the color change among the test areas of the patches that form the test pattern may not be recognized satisfactorily. More specifically, even though the gradation value of the C, M and Y chromatic colorants for the PCBk in the test area is changed from one test area to another, the corresponding color difference among the test areas may not be perceptible. As a result, the output characteristic differences or changes in the print heads of the respective colorants or the printing apparatus may fail to be reflected sufficiently as a color difference among the patches in the test pattern. Thus, in the adjacency comparison the user may not be able to identify a patch that has a greatest color difference from the achromatic color of the reference area, making it impossible to detect a color deviation with high precision.
The object of the present invention is to provide a test pattern printing method, an information processing apparatus and a printing apparatus which can print a test pattern that enables highly precise visual detection of a color deviation caused by output characteristic variations of individual printing apparatus.
In the first aspect of the present invention, there is provided a test pattern printing method of printing a test pattern having a plurality of patches used in correction processing, the correction processing making an output characteristic for each of a plurality of colorants a predetermined one, based on information on a patch selected from among the plurality of patches, to correct a color deviation. The method comprises the step of: printing a plurality of patches, each of which has a test area printed by mixing the plurality of colorants, the test area indicating a degree of the color deviation, the plurality of patches being printed at different mixing ratios of the colorants wherein the mixing ratios of the colorants used to print the plurality of patches are determined so that saturation of the printed patches preferentially changes in accordance with the change in colorant mixing ratio from one patch to another.
In the second aspect of the present invention, there is provided an information processing apparatus for performing a process of printing a test pattern having a plurality of patches used in correction processing, the correction processing making an output characteristic for each of a plurality of colorants a predetermined one, based on information on a patch selected from among the plurality of patches, to correct a color deviation in printing by a printing apparatus. The apparatus comprising means for supplying data to cause the printing apparatus to print the plurality of patches, each of which has a test area printed by mixing the plurality of colorants, the test area indicating a degree of the color deviation, the plurality of patches being printed at different mixing ratios of the colorants wherein the mixing ratios of the colorants used to print the plurality of patches are determined so that saturation of the printed patches preferentially changes in accordance with the change in colorant mixing ratio from one patch to another.
In the third aspect of the present invention, there is provided a printing apparatus capable of printing a test pattern having a plurality of patches used in correction processing, the correction processing making an output characteristic for each of a plurality of colorants a predetermined one, based on information on a patch selected from among the plurality of patches, to correct a color deviation. The apparatus comprises means for printing a plurality of patches, each of which has a test area printed by mixing the plurality of colorants, the test area indicating a degree of the color deviation, the plurality of patches being printed at different mixing ratios of the colorants wherein the mixing ratios of the colorants used to print the plurality of patches are determined so that saturation of the printed patches preferentially changes in accordance with the change colorant mixing ratio from one patch to another.
According to the above configuration, when printing a plurality of patches making up the test pattern used in the color deviation correction processing, the mixture ratio of three colorants for each patch is determined so that a change in the colorant mixture ratio among the patches will preferentially result in a change in saturation. Hence, a change in the colorant mixture ratio among the patches can reflect a color difference among the patches sufficiently.
The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.